As a technique relating to digital audio circuits and their power supply circuits, Japanese Patent Laid-Open Publication No. 2002-223132, for example, discloses an audio reproducing apparatus that includes a control loop for detecting fluctuation of supply voltage of a power amplifier and feeding back the results to a switching regulator and a control loop for feeding forward a PWM signal to control the power amplifier. The audio reproducing apparatus effectively controls the fluctuation of the supply voltage by performing not only feedback control but also feedforward control.
Japanese Patent Laid-Open Publication No. 2004-128662, for example, discloses a digital amplifier that includes a constant voltage power supply circuit for supplying power to an output amplification stage, and a modulation circuit for comparing a PCM multibit digital audio input signal with an output signal of the output amplification stage and modulating the output of the constant voltage power supply circuit based on the comparison results. The digital amplifier reduces distortion caused by the output amplification stage.
However, although these related-art techniques correct fluctuation of the output voltage from the power supply circuit according to the audio output, the frequency of a switching clock signal of a switching regulator of a DC-DC converter used in the power supply circuit is not taken into account.
Japanese Patent Laid-Open Publication No. 9-266424, for example, discloses a technique that changes the voltage of power to be supplied to an amplification circuit, and makes a basic wave component and a harmonic component of noise due to switching of a switching regulator used in a power supply have frequencies different from frequencies of a signal tuned by a tuner unit and a signal obtained by the tuner unit, thereby reducing influence of noise. This technique takes into account the noise in the tuner unit, but no measure is taken against noise in an audio circuit.
FIG. 8 is a block diagram illustrating an exemplary related-art digital audio system.
The digital audio system of FIG. 8 includes a digital audio circuit 101, a DC-DC converter 102 as a power supply circuit, and a speaker SP.
The digital audio circuit 101 includes a first oscillation circuit 111, a digital filter 112, a ΔΣ modulator 113, a D/A converter 114, and an output amplifier 115. The DC-DC converter 102 includes a second oscillation circuit 121 and a control circuit 122.
The DC-DC converter 102 is a power supply of each circuit of the digital audio circuit 101, and outputs an output voltage Vdd generated from an input voltage Vin. The first oscillation circuit 111 outputs a first clock signal CLKA to the D/A converter 114, whereby the first clock signal CLKA is used for D/A conversion. The second oscillation circuit 121 outputs a second clock signal CLKB to the control circuit 122, whereby the second clock signal is used for on/off control of a switch element (not shown) of the control circuit 122.
FIG. 9 is a diagram illustrating exemplary frequency components of noise generated in the output amplifier 115.
As shown in FIG. 9, when the frequency of the first clock signal CLKA output from the first oscillation circuit 111 is 2 MHz and the frequency of the second clock signal CLKB output from the second oscillation circuit 121 is 2.001 MHz, large noise components are generated at 2 MHz, which is the frequency of the first clock signal CLKA, and at 2.001 MHz, which is the frequency of the second clock signal CLKB. A noise component is also generated at 1 kHz, which is the difference between 2 MHz and 2.001 MHz.
The human audible frequency range is considered to be from 20 Hz to 20 kHz. That is, the noise at 2 MHz and 2.001 MHz is not audible to human ears and therefore is not a problem, but the noise at 1 kHz can be heard.
Usually, a circuit such as a crystal oscillation circuit that has very stable oscillation frequency is used as the first oscillation circuit 111 of the D/A converter 114. On the other hand, an inexpensive CR oscillation circuit is often used as the second oscillation circuit 121 of the DC-DC converter 102. The CR oscillation circuit has a frequency that tends to fluctuate in response to changes in drive voltage and temperature and has significant manufacturing variations. Therefore, in the case where the frequency of the first clock signal CLKA is close to the frequency of the second clock signal CLKB, when the absolute value of the difference between the frequencies of the first clock signal CLKA and the second clock signal CLKB is equal to or less than the highest frequency (20 kHz) in the audible frequency range, as mentioned above, the output amplifier 115 outputs a noise signal in the audible frequency range, which can be heard as noise.